1. Field of the Invention
The present invention relates to a high power amplifier mainly used at a cellular phone base station, and a balun circuit for the high power amplifier.
2. Related Art of the Invention
A high power amplifier capable of propagating several tens to several hundreds of watts has recently been used at a digital mobile communication base station. This kind of high power amplifier comprises a plurality of push-pull amplifiers combined in parallel to obtain high power, wherein each push-pull amplifier comprises two identical transistors connected in parallel and supplied with signals 180 degrees out of phase with each other. The explanation of the push-pull amplifier has been given on pages 113 to 116 of xe2x80x9cRadio Frequency Transistorsxe2x80x9d written by Norm Dye and Helge Granberg, published by Butterworth/Heinemann. Therefore, no detailed explanation is given here. This power amplifier circuit requires a power splitter/combiner circuit and baluns at both the input and output of the circuit.
A conventional high power amplifier will be described below referring to FIG. 11. In FIG. 11, the numeral 501 designates a n-way power splitter, the numeral 502 designates a n-way power combiner, the numerals 503, 504 designate n baluns, and the numeral 505 designates n pairs of push-pull amplifiers. A power splitter/combiner circuit used in this configuration will be described below while referring to FIGS. 12 and 13. FIGS. 12 and 13 illustrate the configurations of Wilkinson power splitter circuits. FIG. 12A illustrates a general Wilkinson power splitter circuit. The numeral 601 designates an input terminal, the numeral 602 designates n quarter-wavelength lines, the numeral 603 designates n isolation resistors, and the numeral 604 designates n output terminals. FIG. 12B illustrates a tree configuration of 2-splitters capable of being configured as a plane circuit. The numeral 605 designates an input terminal, the numeral 606 designates two quarter-wavelength lines, the numeral 607 designates an isolation resistor, the numeral 608 designates four quarter-wavelength lines, the numeral 609 designates two isolation resistors, and the numeral 610 designates four output terminals. In addition, FIG. 13 illustrates an asymmetric power splitter type. The numeral 611 designates an input terminal, the numerals 612, 613, 616 and 617 designate quarter-wavelength lines having characteristic impedances different from one another, the numerals 614 and 618 designate isolation resistors, and the numerals 615, 619 and 620 designate impedance transformer circuits. The explanation of the Wilkinson power combiner circuit has been given on pages 205 to 210 of xe2x80x9cFoundations of Microwave Circuits and Applications thereofxe2x80x9d written by Yoshihiro Konishi, published by Sogo Denshi. Therefore, no detailed explanation is given here.
FIG. 14 illustrates the configuration of a conventional balun. The numeral 701 designates an unbalanced terminal, the numeral 702 designates a quarter-wavelength coaxial line having a characteristic impedance of 50xcexa9, the numeral 703 designates an in-phase output terminal, and the numeral 704 designates an opposite-phase output terminal. The explanation of the balun has been given on pages 179 to 181 of xe2x80x9cRadio Frequency Transistorsxe2x80x9d written by Norm Dye and Helge Granberg, published by Butterworth/Heinemann. Therefore, no detailed explanation is given here.
On the basis of the above-mentioned configurations, the conventional high power amplifier has obtained high power for transmission at a base station by combining a plurality of output powers of push-pull amplifiers.
Although the Wilkinson power splitter circuit shown in FIG. 12A can evenly split power into n-way ports at one time, the circuit cannot be attained as a plane circuit. For this reason, the configurations shown in FIGS. 12B and 13 are used generally. The configuration shown in FIG. 12B, however, lends to a large loss in splitter/combiner configuration due to a long transmission path. In addition, the configuration also has a problem of low versatility, because power can be split and propagated only to 2n paths. Furthermore, the configuration shown in FIG. 13 has a problem of unbalanced power splitting, because transmission paths are different lengths from one output to another. Moreover, the balun shown in FIG. 14 is large in circuit size because of the use of the coaxial line, thereby having a problem of difficulty in miniaturization.
In order to solve these problems encountered in the conventional high power amplifier circuit, an object of the present invention is to provide a balun with an incorporated high power amplifier circuit having a drastically reduced circuit size due to use of decreased number of components.
The balun in accordance with the present invention, having been configured by using a coaxial line, is configured by using a multilayer board and by providing coupling lines on the layers above and below a strip line, thereby to obtain two opposite-phase outputs. With this configuration, baluns with power splitter/combiner circuits can be formed on a single multilayer board, whereby the high power amplifier circuit can be miniaturized. In addition, since the balun can propagate two opposite-phase outputs, the number of splits can be reduced, whereby the circuit size of the splitter can be made smaller. In addition, since n-splitting/n-combining can be carried out at one time, loss can be reduced, whereby the efficiency of the power amplifier can be improved.
In order to attain the objects, one embodiment of the present invention offers a balun having:
a multilayer comprised of dielectric layers and conductor pattern layers stacked alternately;
a main line disposed on one of said conductor pattern layers;
a plurality of coupling lines each facing a part of said main line, disposed on one or more of said conductor pattern layers different from said conductor pattern layer on which said main line is disposed, so as to be electromagnetically coupled with said facing part of said main line, and grounded electrically respectively;
an input line disposed on one of said conductor pattern layers; and
output line pairs with respective pairs being connected across corresponding coupling line ends,
wherein one end of said main line is connected to said input line, the other end thereof is electrically grounded, the power of a signal input to said input line is split and propagated to each of said output terminal pairs via each of said coupling lines, and a signal propagated from one output line of each said output line pair is 180 degrees out of phase with the other signal propagated from the other output line.